Potassium channel is a protein which distributes in the plasma membrane of cells and lets potassium ions selectively pass trough it and is considered to be taking an important role in controlling membrane potential of cells. Particularly, this is contributing to the neurotransmission of central and peripheral nerves, pace-making of the heart, contraction of muscles and the like by regulating frequency, persistency and the like of action potential in nerve and muscle cells.
As the classification based on the opening and closing mechanism of the channel, a voltage-dependent potassium channel, an inwardly rectifying potassium channel, a calcium-dependent potassium channel, a receptor coupling type potassium channel and the like have so far been identified. Among them, the voltage-dependent potassium channel has a property to open it when the membrane potential is depolarized. In general, potassium ions are present in a non-equilibrium state of about 5 mM in the extracellular moiety and about 150 mM in the intracellular moiety. Accordingly, when the voltage-dependent potassium channel is opened due to depolarization, potassium ions flow out from the intracellular part into the extracellular part and cause restoration (re-polarization) of membrane potential as a result. Thus, reduction of excitability of nerve and muscle calls is induced accompanied by the opening of the voltage-dependent channel [Non-patent reference 1].
Compounds capable of modifying opening of the voltage-dependent channel have a possibility to regulate various physiological phenomena by regulating excitability of nerve and muscle cells and therefore to become therapeutic drugs of various diseases.
For example, it is known that 4-aminopyridine which is an inhibitor of the A type voltage-dependent potassium channel found in nerve cells causes epilepsy by increasing excitability of nerves [Non-patent reference 3]. In addition, dofetilide which is an inhibitor of HERG potassium channel expressing in the heart, among voltage-dependent potassium channels, is used as an agent for treating arrhythmia based on its property to control excitability of cardiac muscle cells [Non-patent reference 4].
The potassium channel described as SEQ ID NO:2 in Example 1 of U.S. Pat. No. 6,326,168 (corresponding international patent publication pamphlet WO 99/37677) [Patent reference 1] (to be referred to as BEC 1 or BEC 1 potassium channel hereinafter) is a voltage-dependent potassium channel which shows an expression distribution localized to the brain. Its expression is significant particularly in the hippocampus and cerebral cortex. The hippocampus is a region whose relation to memory and learning are strongly suggested [Non-patent reference 5].
Particularly, granule cells of dentate gyrus and CA 1 and CA 3 pyramidal cells wherein BEC 1 potassium channel expresses form a neural circuit, and input of various memories is transmitted from the granule cells of dentate gyrus to the CA 3 pyramidal cell through the CA 1 pyramidal cell, via an excitatory synapse which uses glutamic acid as the neurotransmitter. It is considered that long-term changes in the long-term potentiation, long-term depression and the like synaptic transmission efficiencies found in respective synapses are deeply concerned in the memory and learning. These long-term changes are regulated by the excitation frequency and excitation strength of nerve cells. In addition, the voltage-dependent potassium channel generally has a possibility of being able to control excitability of nerve cells.
Accordingly, it is considered that BEC 1 is concerned in the formation of memory and learning via the excitability control of nerve cells, but this has not been illustratively proved.
A large number of 2,4,6-triamino-1,3,5-triazine derivatives are currently known, and their uses are disclosed as an anti-HIV agent [Non-patent reference 6], an adenosine A 3 antagonist [Patent reference 2], and antimicrobial agents [Non-patent reference 7], [Non-patent reference 8], [Non-patent reference 9] and [Patent reference 3]. Though many potassium channel inhibitors and 2,4,6-triamino-1,3,5-triazine derivatives have so far been reported [Patent reference 3] and [Non-patent reference 10], there are no reports or suggestions stating that they have BEC 1 potassium channel inhibitory action.
The object of the invention is to provide an anti-dementia agent which uses a substance having BEC 1 potassium channel inhibitory action (to be referred to as BEC 1 potassium channel inhibitor hereinafter) as the active ingredient, preferably an anti-dementia agent wherein the BEC 1 potassium channel inhibitor is a 2,4,6-triamino-1,3,5-triazine derivative or a pharmaceutically acceptable salt thereof, a novel 2,4,6-triamino-1,3,5-triazine derivative having BEC 1 potassium channel inhibitory action or a pharmaceutically acceptable salt thereof, and a medicament comprising said novel derivative or a pharmaceutically acceptable salt thereof.
The present inventors have conducted studies with the aim of achieving the above object and found as a result that a BEC 1 potassium channel inhibitor can become an anti-dementia agent. In addition, it was found unexpectedly that a compound having the 2,4,6-triamino-1,3,5-triazine structure has a BEC 1 potassium channel inhibitory action, thus resulting in the accomplishment of the invention.    [Non-Patent Reference 1]            Hille, B. (ed), Ionic Channels of Excitable Membranes (Sinauer Associates, Sunderland, 1992)            [Non-Patent Reference 2]            Catterall, W. A., Chandy, K. G. & Gutman G. A. (eds), The IUPHAR Compendium of Voltage-gated Ion Channels (IUPHAR Media, Leeds, UK, 2002)            [Non-Patent Reference 3]            Yamaguchi, S. and Rogawski, M. A., Epilepsy Res., 11: 9-16 (1992)            [Non-Patent Reference 4]            Gwilt, M., Arrowsmith, J. E., Blackburn, K. J., Burges, R. A., Cross, P. E., Dalrymple, H. W. and Higgins, A. J., J. Pharmacol. Exp. Ther., 256: 318-324 (1991)            [Non-Patent Reference 5]            Levitan, I. B. and Kaczmarek L. K. (1991), The Neuron: Cell and Molecular Biology, Oxford University Press, New York, N.Y.            [Non-Patent Reference 6]            Bioorg. Med. Chem. Lett., (2001) 11, 2229-2234            [Non-Patent Reference 7]            Acta Cienc. Indica. Chem., (1992) 18(4), 405-406            [Non-Patent Reference 8]            Acta Cienc. Indica. Chem., (1985) 11(1), 66-70            [Non-Patent Reference 9]            J. Indian Chemical Society, (1987) 64(12), 770-771            [Non-Patent Reference 10]            J. Inst. Chem. (India), (1987) 59(4), 183-185            [Patent Reference 1]            U.S. Pat. No. 6,326,168            [Patent Reference 2]            JP-A-11-158073            [Patent Reference 3]            International Publication Pamphlet WO 99/1442        